Voids between structural components have been a common problem since the start of aviation. This has consistently been caused by the manufacturing tolerances of the components in question not being sufficiently tight to allow for a good interface to be made. This situation has been made significantly worse by the introduction of composite materials where some of the manufacturing processes give downgraded tolerance performances.
Currently there are a number of ways to solve this problem. All of which are time consuming and expensive. These are:
1. Assembling the components, measuring the void and fixing packing pieces into the measured void. Any packing pieces have to be bonded or mechanically fastened in position to ensure they do not detach in service.
2. Applying a structural filler (liquid shim) to the joint during assembly, ensuring it fully covers the void with minimal air pockets. When applied the excess material that has extruded out of the sides has to be cleared up and then the shim requires a significant amount of time to harden (cure), during which time the components cannot be moved. Liquid shim can typically only be used to fill voids of less than around 1.25 mm.
3. Including within one of the components an excess of material which is fettled/filed to match the surface of the other component on assembly, again taking significant time by a skilled workforce.
4. In the case of laminate composite components, if these are found to be of insufficient dimension upon assembly to other components, additional plies can be added after the component has been cured. This then requires a re-cure of the component to incorporate the additional plies. This is costly and significantly extends the assembly time.